From the prior art, it is known to determine the oxygen content in gas mixtures using amperometric gas sensors based on solid-state electrolytes. The solid-state electrolyte functions as a substrate and has arranged thereon electrodes in the form of a cathode and an anode between which a DC voltage can be applied by means of a voltage source. The gas mixture whose oxygen content is to be determined surrounds the two electrodes and the solid-state electrolyte. The electrodes, together with the solid-state electrolyte and the surrounding gas mixture, each form what is known as a “three-phase boundary.” At the three-phase boundary of the cathode, the oxygen molecules are ionized and incorporated into vacancies in the lattice of the solid-state electrolyte. Application of a voltage causes a current to flow through the electrodes and the solid-state electrolyte. Charge transfer is via the vacancies in the lattice of the solid-state electrolyte. At the three-phase boundary of the anode, a reverse reaction takes place and the recombined oxygen molecules are released into the gas mixture.
In order to determine the oxygen concentration in the gas mixture, a so-called “limiting current behavior” of the current-voltage characteristic is needed because in the limiting current range, the absolute magnitude of the resulting current, on the one hand, is nearly independent of temperature and voltage and, on the other hand, is linearly dependent on the oxygen concentration in the gas mixture. The limiting current flowing between the electrodes upon application of a DC voltage is measured by a suitable measuring device, and thus provides a measure of the oxygen content of the gas mixture. In order to ensure a suitable limiting current behavior of the current-voltage characteristic, it is necessary to limit, by selective intervention, the current along the current path cathode—transition cathode/solid-state electrolyte—solid-state electrolyte—transition solid-state electrolyte/anode—anode. The measure commonly used for this purpose is to limit the flow of oxygen molecules to the cathode by means of a diffusion barrier. In this regard, reference is made, for example, to DE 20 2004 015 400 U1. In certain sensor designs, however, it is problematic to provide a sufficiently thick diffusion barrier of this kind because of the production technology used.